Field of the Invention
This invention is concerned with a system which can achieve communication links between a plurality of spatially separated entities. In this sense, the device to be described herein could be applied to any circumstance which required this type of function. However, there is a specific context of application in which the capabilities of the invention are most clearly demonstrated, and in which its utility and advantages over prior art can be most clearly presented. This said context is that of a telephone central office switching system, and therefore it is within this context that the use of the invention will be recommended, and its advantages over prior art pointed out.
It is the basic function of a telephone switching system to achieve a communications link between two or more telephones. In the performance of this function, various automatic systems have evolved; but all of these systems can be grouped into two general categories:
I. space division systems: which use physically distinct paths to achieve communication between telephones; PA1 Ii. time division systems: which use a time-shared common path between telephone terminations at the switching system itself. PA1 1. electro-mechanical PA1 2. electronic PA1 a. Adaptability of the system to different operating requirements: This adaptability arises from the fact that the control functions in an electronic systems are not "hard-wired," but rather are a part of a stored program, and hence machine operation can be altered simply by changing the program. PA1 b. If it became desireable to increase the size of a given installation, much easier implementation of this was available with an electronic system.
This discourse will first concern itself with the different types of space division systems. These can be divided into two general catagories:
The electro-mechanical systems, of course, preceded the electronic systems, and even some of the earlier types of electro-mechanical systems are still in widespread use today. The first type of electro-mechanical system that was of major significance was the step-by-step system. This system used banks of the Strowger rotary-type switch along with the appropriate supportive equipment to achieve the required switching functions.
Increased numbers of subscribers posed problems of maintaining satisfactory speed in the switching process itself, and of dealing adequately with the inherent complexity of a larger sized system. As a result, a number of other electro-mechanical systems evolved. These systems used various switching arrangements, and various types of electromechanical components. The crossbar system is one such system, and features a trend that was developed extensively in the later electronic systems: that of common or centralized control of the switching process.
The search for a system that could handle ever increasing numbers of subscribers and traffic loading conditions, and also possessed the possibility of fulfilling long term anticipated increased subscriber demands, led to the development of electronic switching systems. Systems of this type feature an electronic central processor which functions as a central supervisory control unit for the entire switching system. The #1 ESS, developed by Bell Telephone Laboratories, is a good representative example of such a system. In addition to an electronic centralized control, this system featured a new switching element, the ferreed cross-point switch. This switch had magnetic retentive properties which allowed it to remain in a closed position upon the application of a momentary pulse. This fact facilitated its use as a switching element within the network of the system. Also of significance was the ESS technique for detecting the off-hook status of a subscriber telephone. Off-hook detection is one of the basic functions that must be performed by a telephone switching system. The ESS utilized a new type of magnetic sensing device, the ferrod sensor, which reacted to a change in impedance of the subscriber line. Banks of these sensors were used, and they were repeatedly scanned by the central processor unit to detect an off-hook status.
The electronic systems outperformed their electromechanical counterparts in a number of areas:
1. They had improved speed, which is very important for a large and complex system.
2. They had improved versatility. This was realized in two areas:
3. Their overall traffic handling capability was increased.
4. Maintenance features were improved due to diagnostic routines included in the central processor programs.
On an initial cost basis, electronic systems tend to exceed the cost of electro-mechanical systems, but it is widely held that this fact is adequately compensated for by the long term advantage of flexibility and low maintenance cost provided for by an electronic switching system.
Upon observing the above cited systems, it can be seen that there have been a series of improvements which have increased both the speed and versatility of the systems. Among these improvements are more durable and more sophisticated components, and the use of centralized electronic control. Nevertheless, all of these systems actually achieve telephone-telephone interconnection by closing the proper combination of electro-mechanical switching elements, which individual elements are part of a switching network. The use of individual switching elements structured into a network results in an office which is very complex for any system of appreciable size. Furthermore, the whole approach of achieving central office function in this way encounters inherent problematic factors. The following paragraphs will discuss these factors.
Upon considering the relationship between the central control of a switching system and the switching network itself, a number of factors must be taken into account for both electro-mechanical and electronic systems.